1. Field of the Invention
Embodiments of the present invention relate generally to display systems, and, more particularly, to a system and method for projection display of an image.
2. Description of the Related Art
Digital projection is used in televisions, projectors and video projectors. In digital projectors, a projected image is created by illuminating a spatial light modulator, such as a liquid-crystal-on-silicon (LCOS), a high-temperature polysilicon transmissive panel (HTPS) a deformable micro-mirror device (DMD), or other microelectromechanical system (MEMS) device. A deformable micro-mirror device is an array of small mirrors arranged in a typically rectangular matrix on a semiconductor chip, known as a DMD chip, or DMD panel. Each mirror represents one pixel in the projected image, and therefore the resolution of the projected image corresponds to the number of micro-mirrors contained in the DMD panel. These mirrors are repositioned rapidly to reflect light either through projection optics for viewing, or to a light dump. Rapidly toggling a mirror between these two orientations, i.e., “on” and “off”, produces gray scales for one of the pixels, where the brightness of the gray scale is controlled by the ratio of on time to off time. To display color images, the mirrors of the DMD panel are positioned for each primary color as required in rapid succession. The changes in mirror position for each primary color are synchronized with a multi-color light source, and variation in intensity of each primary color at an image pixel is controlled by rapidly toggling the mirror corresponding to the pixel between the on and off orientations.
A significant cost of digital projection systems is the spatial light modulator, such as a DMD panel. For higher-resolution digital projection systems, which require more pixels to provide a higher-resolution image, spatial light modulator cost is a limiting factor in the cost-effective production of such systems. This is because the cost of a spatial light modulator increases non-linearly as the resolution of the spatial light modulator is increased.
One approach in the art for creating a high-resolution display is known as wobulation. Wobulation increases the resolution of a digital projection system without a corresponding increase resolution of the system spatial light modulator. Wobulation accomplishes this by sequentially overlaying two or more lower-resolution images to effectively produce an image with a higher-resolution. In this approach, the image data making up a high-resolution image are essentially divided into multiple lower-resolution images, or subframes, which are displayed in rapid succession to appear to the human eye as if projected simultaneously and superimposed. When displayed for viewing, each subframe is displaced from the other subframes making up the original image by a fraction of a pixel, e.g., one-half pixel width. To the human eye the subframes appear to be superimposed on each other, so the optical information contained in the multiple subframes is effectively combined to produce a close facsimile of the original high-resolution image. Because each subframe is by itself a low-resolution image, and because each subframe is generated by the same spatial light modulator, wobulation allows a digital projection system to emulate the spatial resolution of a higher-resolution display device at a reduced cost. A more detailed description of wobulation may be found in U.S. Pat. Nos. 7,030,894 and 7,034,811.
The displacement of the subframes that allows wobulation of a high-resolution image is generally accomplished by physically shifting the optical path for each subframe by the desired amount, i.e., a fraction of the width of one pixel. In some projection systems that use wobulation in conjunction with a DMD panel, a prism is positioned between the DMD panel and projection lens of a digital projection system, and is mechanically translated to a unique position for the projection of each subframe. In this way, each subframe is projected along a slightly different optical path, resulting in each subframe being offset from each other subframe and producing a wobulated image that approximates the resolution of the original. In other systems, the optical path of each subframe is altered by translating the DMD panel. In either case, wobulation relies on the precise mechanical displacement of a digital projection system component that takes place at very high speed, and such high-speed mechanical displacement is subject to drift, misalignment, non-linear behavior, and other issues that can affect image quality.
In addition, a digital projection system using wobulation requires control electronics and image processing drivers having substantially greater bandwidth than a digital system displaying the same resolution image without the use of wobulation. The greater bandwidth is required since wobulation produces two complete images for a single image frame and therefore must rely on image oversampling, i.e., the generation of images at twice the normal frequency of a non-wobulated digital display system.
Accordingly, there is a need in the art for methods and apparatus for displaying an image that increases the effective resolution of a display device without relying on high-speed mechanical displacement of optical components or a higher bandwidth spatial light modulator.